1. Cooling and Crystallization:
* As a magma body cools, minerals with higher melting points (like olivine, pyroxene, and calcium-rich plagioclase) start to crystallize first.
* These early-formed crystals are denser than the remaining melt and settle to the bottom of the magma chamber.
2. Removal of Crystals:
* The settling crystals are physically removed from the remaining melt, either by:
* Gravity settling: The denser crystals sink to the bottom of the magma chamber.
* Filter pressing: As the magma chamber cools and solidifies, the remaining melt can be squeezed out, leaving behind a concentrated pile of crystals.
3. Enrichment of Residual Melt:
* The removal of early-formed crystals enriches the remaining melt in elements that were not incorporated into those minerals.
* This leads to a change in the chemical composition of the magma. For example, removing olivine and pyroxene will enrich the melt in silica, sodium, and potassium.
4. Continued Crystallization and Differentiation:
* As the magma continues to cool, different minerals crystallize out in sequence, each removing specific elements and further changing the composition of the remaining melt.
* This process can continue through multiple stages, leading to the development of a range of magma compositions within the original magma body.
Examples of Fractional Crystallization:
* Layered Intrusions: These are large bodies of igneous rock that exhibit distinct layering due to the sequential crystallization and settling of different minerals.
* Bowen's Reaction Series: This is a classic model that illustrates the order of mineral crystallization and the resulting compositional changes in a cooling magma.
Fractional Crystallization is Crucial for Magmatic Differentiation:
* It allows for the generation of diverse igneous rocks from a single parent magma.
* It plays a role in the formation of different rock types, including:
* Basalt: A mafic rock formed from the initial crystallization of olivine and pyroxene.
* Andesite: An intermediate rock formed from the later crystallization of plagioclase and amphibole.
* Rhyolite: A felsic rock formed from the final crystallization of quartz, alkali feldspar, and other minerals.
Fractional crystallization is a fundamental process in understanding the formation and diversity of igneous rocks, and it highlights how the evolution of magma bodies can lead to a wide range of geochemical and petrological variations.
